In response to the need of society at large for the conservation of natural resources and energies, various methods have been studied by car manufacturers to reduce the fuel consumption of running automobiles. One of the approaches under current review is to make automotive parts from materials other than metal or steel. In practice, attempts are being made to minimize the weight of a car by replacing metal with lighter materials such as high-tensile steel, aluminum and plastics in as many parts as possible. Car exterior parts such as bumpers have heretofore been made of metal materials which are painted to provide corrosion resistance and good appearance. Plastic parts are corrosion-resistant by themselves, but as such their appearance is not as good as the appearance of painted metal parts. To solve this problem, coloring of plastic parts is currently under review, but because of the inherently low thermal deformation temperature of plastics compared with metal, the latitude for the selection of suitable coloring paints is fairly limited.
Metals which generally have high surface energies are labile in their elemental form and become stable when an energy-lowering oxide film forms on the surface as a result of adsorption of aerial moisture or oxygen or acidic gases. On the other hand, plastics having low surface energies have poor paint receptivity. Among various plastics known today, polypropylene is characterized by particularly low paint receptivity because it is highly crystalline, has a large angle of contact with water, has a low surface energy and is nonpolar. Therefore, in order to provide plastics with improved adhesion to paint coating, a special surface treatment is necessary before paint application.
A number of methods have been proposed for effecting such preliminary treatment of plastics and they include primer application, plasma treatment, irradiation with ultraviolet rays, chromate treatment, flame treatment, electric discharging, and exposure to radiation. Of these methods, only the primer application has been commercialized. however, there are not many manufacturers of suitable primer paints and, in addition, the production cost and sales prices of such primer paints are still high. In order to overcome these disadvantages, studies have been conducted into replacing the method of primer application by plasma treatment. As shown in Unexamined published Japanese Patent Application Nos. 147432/1983 and 147433/1983, active efforts are underway principally for the purpose of modifying the surface of polyolefinic resins such as polyethylene and polypropylene. Achieving the plasma treatment to a surface of the plastic material is also shown in "Plastics Engineering", page 41, (October, 1985) and in "Journal of Applied Polymer Science" Vol. 11, page 1461, (1967).
Plasma treatment is a dry method that employs either oxygen (to effect surface oxidation) or an inert gas as a plasma source. Unlike the method of primer application, plasma treatment is capable of preliminary treatment of plastic articles at low cost. However, this method requires the operator to check whether satisfactory plasma treatment has been achieved. Conventionally, the checking has been accomplished by performing, after paint application, a peel test (or adhesion test) (JIS K 6829) on the paint or a surface tension test on the surface of the plastic article (in which the change in its surface tension with a wetting reagent is determined). However, these methods have the following problems: because of the complexity of the checking procedures, samples have to be extracted from each lot of the products or the production line has to be stopped for each checking; even if defective products are found, the lot from which they have been sampled is already in the stage of subsequent steps; the results of checking are not quantitative; and great skill is required for ensuring reliable checking.
With a view to eliminating these problems, it has been proposed to apply a substance that will change properties or color upon plasma treatment to a plastic article and to check the degree to which the article has been subjected to plasma treatment by evaluating the degree by which the substance has changed color as a result of plasma treatment. In this method, a phthalocyanine dye may be used as the substance that changes color upon plasma treatment; when subjected to treatment with an oxygen plasma in a treatment bath, the dye is excited to produce a pink color, which is retained for about 10 minutes even after it is recovered from the bath and gradually turns red purple, then blue purple, and finally blue. Therefore, by observing such changes in color, the degree of plasma treatment applied to the plastic article can be checked.
The method described above has several defects. For one thing, if a substance that changes in properties or color upon plasma treatment is simply attached to the plastic article, it sometimes evaporates from the surface of the article during plasma treatment and fails to provide satisfactory results in subsequent checking. Secondly, even if the plastic article has good dyeability, with the dye being deposited on it in a sufficient thickness to avoid its loss due to evaporation, the actual change that has occurred in the dye cannot be fully checked if its coat has a smooth surface. No matter how much dye is deposited on the surface of the article, it is only the dye present in the surface of the dye layer that changes color upon plasma treatment and the color of the unreacted dye in the deeper area of the dye layer can be seen through its surface, which makes it practically impossible to selectively observe the color change that has occurred in the surface layer of dye.